Phase-locked loop (PLL) frequency synthesizers may be used as building blocks in many types of systems, such as audio and video processing systems, communication systems and in other systems, such as processing systems. A PLL is a device which generates an output signal with an output frequency that is a function of the frequency of a reference signal. When a PLL is implemented in a device, the frequency of the output signal may frequently change. For example, the frequency of the output signal may change at the start-up of the device or when the device changes from one channel to another.
The PLL may include certain components connected in a feedback loop. For example, the PLL may include a voltage controlled oscillator (VCO), a phase frequency detector (PFD) and a loop filter. The PLL may additionally include a feedback frequency divider in applications where the VCO frequency is designed to be a multiple of the reference frequency.
The PFD may control the frequency of the output signal of the VCO. The PFD in the PLL receives the output signal in the feedback loop and compares the frequency of the output signal to the frequency of the reference signal. Based on the comparison of the frequency of the output signal to the frequency of the reference signal, the PFD generates a control signal that is provided to the low-pass filter and then to the VCO in order to control the frequency of the output signal of the VCO.
However, there may be a large variation in the loop bandwidth of the PLL. In addition, the PLL settling times and noise performance may severely deteriorate. Therefore, benefits may be realized by providing improved phase locked loop filters.